The strike in the province of al-Bayda targeted vehicles the insurgents were travelling in at the time.

Some reports say three civilians in another car were also killed.

The exact nature of the strike is unclear, but the US has launched several drone attacks on militants in Yemen in the past.

Al-Bayda has seen serious security incidents recently, and earlier this week suspected al-Qaeda militants shot dead the deputy governor as he was leaving his home.

Yemen's Defence Ministry website quoted a security source as saying a number of "terrorists" had been killed in the air strike.

"An air strike targeted cars that suspected al-Qaeda militants were in and killed 13 of them in the Sawma'a area of al-Bayda," a security source told Reuters.

The US has stepped up drone strikes as part of a campaign against al-Qaeda in the Arabian Peninsula (AQAP), which is based in Yemen.

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The US has faced criticism over its drone programme by human rights groups

Yemen is among a handful of countries where the US acknowledges using drones, although it does not comment on the practice.

Last month, Yemen's President Abdrabuh Mansur Hadi defended the use of drones in the country, saying they "have greatly helped in limiting Al-Qaeda activities, despite some mistakes, which we are sorry about."

The drone programme has come under criticism from human rights activists concerned over civilian casualties.

The United Nations said 16 civilians were killed and at least 10 wounded when two separate wedding processions were targeted in December.

Local security officials said the party had been mistakenly identified as al-Qaeda members.

Following the deaths, Yemen's parliament voted for a ban on drone strikes, but analysts say lawmakers are unlikely to be able to halt the US campaign.

AQAP is considered to be the most dangerous branch of al-Qaeda in the world and it overran much of Yemen's south in 2011.

A video posted online earlier this week purported to show AQAP leader Nasser al-Wuhayshi addressing more than 100 members of the group at a mountainous location in al-Bayda.

In the film, thought to have been shot near the border with Shabwa province, al-Wuhayshi pledges to pursue the war against Western "crusaders."